1. Field of the Invention
The present invention relates to an apparatus for recording and reproducing optical information and a prism coupler used as an optical integrating means in an optical disk memory, etc.
2. Description of the Related Art
In a general apparatus for recording and reproducing optical information and having a means for detecting light by an optical waveguide, light is converged onto an optical disk through a focusing grating coupler and light reflected from the optical disk is again guided to an optical waveguide layer through the focusing grating coupler. Accordingly, light utilization efficiency provided by using an optical pickup is influenced by diffraction efficiency (optical coupling efficiency) of the focusing grating coupler.
In general, incident and light-emitting coupling efficiencies of the focusing grating coupler are respectively set to 20% and 50% so that the focusing grating coupler has low incident and light-emitting coupling efficiencies. When coupling efficiency of waveguide light from a semiconductor laser to the optical waveguide layer is set to a/100 (%) in consideration of the light utilization efficiency, the transmittance of a waveguide beam splitter is equal to 50%. Accordingly, a quantity of light reaching the optical disk and a quantity of light reaching a signal detecting optical system are respectively set to small values such as 10% and 0,625% even when the above value a is set to one. Accordingly, in the optical pickup used for an optical disk memory, it is necessary to increase the quantities of light reaching the optical disk and the signal detecting optical system as much as possible. However, no structure for increasing these light quantities as much as possible is sufficiently considered in the above general optical information recording/reproducing apparatus.
In a prism coupler as a second general example, a laser beam is incident to a tapered region of a gap layer having a tapered portion having about 1 mm in length. Accordingly, it is necessary that the laser beam having a very small beam radius such as 0.158 mm is incident to this tapered region. Therefore, in such a structure of the prism coupler, only the laser beam having a small beam radius equal to or less than several millimeters divided by ten can be coupled to an optical waveguide layer. Further, it is necessary to make such a very small laser beam incident to the tapered portion of the gap layer. Therefore, an accuracy of about .+-.10 .mu.m is required in an incident position of the laser beam so that it is very difficult to adjust this incident position. Further, optical coupling efficiency is greatly changed by inclination of the tapered portion so that it is necessary to strictly control the inclination of the tapered portion, which is troublesome in operation of the prism coupler.